Conventional techniques for producing a heat-generating element that generates heat through reaction of an oxidizable metal with oxygen of air include, for example, the method for producing a heat generating element described in Patent Literature 1 below. This method includes applying an ink-like or creamy heat-generating composition to a base material sheet (covering material). The heat-generating composition is prepared by sequentially putting activated carbon, a thickener, a surfactant, a pH modifier, edible salt, and iron powder into a mixer in that order, stirring the mixture, and further kneading the mixture while adding water.
Patent Literature 2 below discloses a process for producing a heat-generating composition containing essentially iron powder, a carbon ingredient, a reaction accelerator, and water. The process includes providing a horizontal cylindrical mixer equipped with a rotatable screw, sequentially putting ingredients of the composition, such as iron powder and a carbon ingredient, in the cylindrical mixer and conveying the ingredients to a subsequent step while mixing by the rotation of the screw.